In many cases, photons are used as information carriers in quantum cryptography communication or quantum computation. The methods for generating such a photon include a conventional laser, a spontaneous emission of quantum dots or coupled quantum dots, and a parametric down-conversion using a non-linear optical crystal.
In quantum cryptography communication, particularly in quantum key distribution (QKD), a photon, which is a quantum information carrier, is transmitted through an optical fiber. Usually, one photon carries one quantum bit, which is a unit for quantum information. There are several possible methods for generating such a photon.
First, a method for using weak coherent light due to a conventional laser can be cited. Second, a method for using a spontaneous emission of quantum dots can be cited. Third, a method for using a spontaneous emission of coupled quantum dots can be cited. And fourth, a method for using parametric down-conversion utilizing a non-linear optical crystal can be cited.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-066463    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-335503    Non-Patent Document 1: P. G. Kwiat et al., Physical Review Letters, vol. 75 p. 4337 (1995)    Non-Patent Document 2: G. S. Solomon et al., Physical Review Letters, vol. 86, p. 3903 (2001)    Non-Patent Document 3: C. H. Bennett et al., Journal of cryptology, vol. 5, p. 3 (1992)